Proteins produced in various host cells are used in a wide variety of applications. For example, such proteins are employed for experimental purposes such as determination of crystal structure and as antigens for raising antibodies. They are also used for an increasing number of therapeutic purposes. In general, the technology for producing proteins in host cells relies on polynucleotides referred to as expression vectors. These vectors are typically circular pieces of DNA known as plasmids, which may include various genetic elements to facilitate the expression of a protein coding sequence. Such elements typically include a promoter and may also include elements such as downstream transcription termination signals, polyadenylation signals, etc. In order to express a protein of interest, a polynucleotide that includes the coding sequence for the protein is inserted into the expression vector, e.g., at a specific location. The resulting vector is then introduced into host cells, which then express the protein of interest.
A wide variety of different host cells, ranging from prokaryotic cells such as Escherichia coli to eukaryotic cells, such as fungal (e.g., yeast), insect, mammalian, and plant cells are used for the production of proteins. The choice of host cell can be extremely important. For example, different host cells may synthesize the protein either more or less efficiently, which affects the final yield of product. In addition, certain proteins are soluble in certain host cells but insoluble in others. Proteins produced in eukaryotic cells are subject to a variety of post-translational modifications that do not occur in prokaryotic cells and may be needed for functional activity.
In general, different types of host cell frequently require different promoters. For example, many promoters that are utilized by prokaryotic host cells are inactive in eukaryotic hosts. Thus expression vectors are typically designed for use in a single host or class of hosts and contain appropriate genetic elements such as promoters for expression in that host or class of hosts. In order to express a protein of interest in multiple cell types, it is therefore typically necessary to construct multiple different expression vectors, each containing a sequence encoding the gene of interest and a promoter appropriate for expression in a different host cell. This can be inconvenient and time-consuming, particularly when there is a need to rapidly test multiple proteins in order to identify an appropriate host cell. Accordingly, there is a need in the art for a promoter that would function in multiple cell types and a need for an expression vector system that would allow expression of a protein of interest in multiple cell types. In addition, there is a need in the art for high throughput screening systems for proteins using multiple host cell types.